While you're tire shopping, have a look at my forum post on Fuzion tires. I bought mine a month ago for my SC430. They are fantastic, and $125 per tire at TireRack, compared to $225 for Michelin Pilot Sport 2. Additionally, they should wear longer, are very comfortable, have great handling characteristics in wet and dry conditions. They are an ultra-performance tire, and while they will not quite match the PS2 for handling, they definitely come close... I doubt many SC430 owners would be able to discern the difference. I like the Fuzion, too, because of its extra rubber protection on the sidewall, designed to save rims from curb damage. Fuzion is manufactured by Bridgestone.
In addition to ride quality, another advantage of swtiching FROM run-flats is that run-flat tires cannot be repaired, as a standard tire can. Your only option is to replace it, and you'll likely have to do that in pairs.
The best solution to the spare tire issue, in my mind, is still Lexus Care (free towing), AAA, and a road hazard protection package when you buy your tires.
Steve
San Diego, 2002 Silver SC430
Got 27,000 miles from original run-flats